1. Computer and Network Security
CSEN 1001
Computer and Network Security
Amr El Mougy
Alaa Gohar
Heba Anwar
**Slides are attributed to William Stallings

2. Symmetric Ciphers – cont’d
Lecture (4)
Symmetric Ciphers – cont’d

3. Modes of Operation Block ciphers encrypt fixed size blocks
eg. DES encrypts 64-bit blocks with 56-bit key
Need some way to en/decrypt arbitrary amounts of data in practise
ANSI X Modes of Use (now FIPS 81) defines 4 possible modes
Subsequently 5 defined for AES & DES
Have block and stream modes

4. Electronic Codebook Mode (ECB)
Message is broken into independent blocks which are encrypted
Each block is a value which is substituted, like a codebook, hence name
Each block is encoded independently of the other blocks
Ci = DESK1(Pi)
Uses: secure transmission of single values

5. Limitations of ECB Message repetitions may show in ciphertext
if aligned with message block
particularly with data such graphics
or with messages that change very little, which become a code-book analysis problem
Weakness is due to the encrypted message blocks being independent
Main use is sending a few blocks of data

6. Cipher Block Chaining Mode (CBC)
Message is broken into blocks
Linked together in encryption operation
Each previous cipher blocks is chained with current plaintext block, hence name
Use Initial Vector (IV) to start process
Ci = DESK1(Pi XOR Ci-1)
C0 = IV
Uses: bulk data encryption, authentication

7. Limitations of CBC A ciphertext block depends on all blocks before it
Any change to a block affects all following ciphertext blocks
Need Initialization Vector (IV)
which must be known to sender & receiver
if sent in clear, attacker can change bits of first block, and change IV to compensate
hence IV must an unpredictable value
can be sent encrypted in ECB mode before rest of message

8. Cipher Feedback Mode (CFB)
Message is treated as a stream of bits
Added to the output of the block cipher
Result is feed back for next stage (hence name)
Standard allows any number of bit (1,8, 64 or 128 etc) to be feed back
denoted CFB-1, CFB-8, CFB-64, CFB-128 etc
Most efficient to use all bits in block (64 or 128)
Ci = Pi XOR DESK1(Ci-1)
C0 = IV
Uses: stream data encryption, authentication

9. Limitations of CFB Appropriate when data arrives in bits/bytes
Most common stream mode
Limitation is need to stall while doing block encryption after every n-bits
Note that the block cipher is used in encryption mode at both ends
Errors propagate for several blocks after the error

10. Output Feedback Mode (OFB)
Message is treated as a stream of bits
Output of cipher is added to message
Output is then feed back (hence name)
Feedback is independent of message
Can be computed in advance
Ci = Pi XOR Oi
Oi = DESK1(Oi-1) i>1
O1 = DES(Nonce)
Uses: stream encryption on noisy channels

11. Limitations of OFB Bit errors do not propagate
More vulnerable to message stream modification
A variation of a Vernam cipher
hence must never reuse the same sequence (key+IV)
Sender & receiver must remain in sync
Originally specified with m-bit feedback
Subsequent research has shown that only full block feedback (i.e. CFB-64 or CFB-128) should ever be used

12. Counter Mode (CTR) Relatively “new” mode, though proposed early on
Similar to OFB but encrypts counter value rather than any feedback value
Must have a different key & counter value for every plaintext block (never reused)
Ci = Pi XOR Oi
Oi = DESK1(i)
Uses: high-speed network encryptions

13. Limitations of CTR Efficiency Random access to encrypted data blocks
can do parallel encryptions in h/w or s/w
can preprocess in advance of need
good for bursty high speed links
Random access to encrypted data blocks
Provable security (good as other modes)
But must ensure never reuse key/counter values, otherwise could break (cf OFB)

15. Problem 1
In the textbook the four basic modes of operations of block ciphers (ECB, CBC, OFB, CFB) are analyzed with respect to error propagation in encryption. That is, the consequences on ciphertext blocks by changing a single plaintext block are discussed.
For all four modes of operation analyze the effect on the decryption of remaining blocks if for the sequence of ciphertext blocks c1, c2, ….., cn some ciphertext block cj is erroneous, 1 ≤ j < n. That is, specify which of plaintext blocks xj , xj+1, xj+2,……., xn are received correctly.
Solution:
ECB mode: Only x1 decrypted incorrectly.
CBC mode: Only x1; x2 decrypted incorrectly.
OFB mode: Only x1 decrypted incorrectly.
CFB mode: Error propagates while bad data is still in the shift register.

16. Problem 2
Suppose that we use DES in cipher block chaining (CBC) mode. The encryption rule for message Mi, key K and cipher Ci is:
𝐶 𝑖 =𝐷𝐸𝑆 𝑀 𝑖 ⊕ 𝐶 𝑖−1 ,𝐾 𝑖=1, 2, ……
where C0 is an initial block and ⊕ is the XOR operation.
What is the decryption rule (i.e. what is 𝑀 𝑖 =………)? (You can alternatively draw a block diagram).
Suppose an attacker changes Ci into 𝐶 𝑖 ′ ≠ 𝐶 𝑖 . How many messages are then decrypted incorrectly?
b) Suppose that we use DES in counter (CTR) mode. The encryption rule for key K and message Mi, at time i > 0 is
𝐶 𝑖 =𝐷𝐸𝑆 𝑅 𝑖 ,𝐾 ⊕ 𝑀 𝑖 𝑅 𝑖 = 𝑅 𝑖−1 +1
where R0 = some starting value and ⊕ is the XOR operation.

17. Solution
𝑀 𝑖 =𝐷𝑒𝑐𝑟𝑦𝑝𝑡𝑖𝑜𝑛 𝐶 𝑖 ,𝐾 ⊕ 𝐶 𝑖− 𝑖=1, 2, …… The messages decrypted incorrectly are i and i + 1
𝑀 𝑖 =𝐸𝑛𝑐𝑟𝑦𝑝𝑡𝑖𝑜𝑛 𝑅 𝑖 ,𝐾 ⊕ 𝐶 𝑖 𝑅 𝑖 = 𝑅 𝑖− The message decrypted incorrectly is only i

18. Confidentiality using Symmetric Encryption
Have two major placement alternatives
Link encryption
encryption occurs independently on every link
implies must decrypt traffic between links
requires many devices, but paired keys
End-to-end encryption
encryption occurs between original source and final destination
need devices at each end with shared keys

20. Placement of Encryption
When using end-to-end encryption must leave headers in clear
so network can correctly route information
Hence although contents protected, traffic pattern flows are not
Ideally want both at once
end-to-end protects data contents over entire path and provides authentication
link protects traffic flows from monitoring

21. Placement of Encryption
Can place encryption function at various layers in OSI Reference Model
link encryption occurs at layers 1 or 2
end-to-end can occur at layers 3, 4, 6, 7
as move higher less information is encrypted but it is more secure though more complex with more entities and keys